The invention relates to unidirectional or bi-directional DC/AC converter. This includes switching power amplifier, AC power source, frequency converter, line conditioner and uninterruptible power source.
Many terms exist to describe various types of devices used for power conversion. The following definitions are provided in order to avoid any conflict of terms. A switching power supply (SPS) is an AC/DC or DC/DC converter. A switching power amplifier (SPA) is an AC/AC or DC/AC converter. An SPA that produces a fixed frequency is commonly referred to as inverter, AC voltage regulator, AC power source, line conditioner, frequency converter, etc. An SPA that amplifies a variable frequency is often narrowed to class-D amplifier, whereas other techniques exist. An uninterruptible power source/supply/system (UPS) is a bi-directional DC/AC converter. The UPS charges a battery when line is present and simulates line voltage when line fails. In the following disclosure, the term converter refers to a block performing power conversion within a parent apparatus.
Conventional SPA and UPS each comprise an output inductor that continuously delivers a current to an output capacitor. Moreover, a feedback signal introducing delay and phase shift is used to determine that current. Only an optimal level of the current is established. In particular, a rate at which the current is regulated is very limited in order to maintain high stability. However, variations of load impedance over amplitude and frequency are often rapid and unpredictable. A precise correction is simply impossible since, at the end of every switching cycle, the correction is either insufficient or continues while no longer required. In order to minimize an output voltage ripple, a powerful output filter is used. However, this further contributes to unpredictability of load impedance. Unless a well-behaved load is used, the high accuracy is unattainable with traditional techniques. This includes most sophisticated class-D amplifiers. Usually, the accuracy of the output voltage produced by the UPS is nonessential. However, an excessive switching results in reduced efficiency. During the battery charging, the UPS acts like an SPS.
Class-D amplifiers are plagued by numerous inherent flaws. Pulse width modulation (PWM) and other modulation schemes are used to average the input and feedback signals. Moreover, the output voltage is obtained by averaging the pulse train in the output filter. The result is slow response time and sluggish performance. The output voltage is never on target as the output inductor xe2x80x9cdragsxe2x80x9d it around. Even if the output voltage is precisely equal to the input voltage, multiplied by gain, class-D amplifier will overshoot or undershoot. The output voltage is thus corrected and/or falsified in every switching cycle. Moreover, increased cost and reduced efficiency of class-D amplifiers are unacceptable. No-load operation can produce large resonant currents in the output filter. Minimum load matching the main load is often required. Problems associated with power devices offer many other examples. In order to correct even a tiny overshoot at peak current, the output inductor has to be fully discharged and fully recharged. The result is increased response time and overblown current ratings. But even the switching itself is troublesome. Dead time is excessive in order to take into account temperature dependence and device differences. Output capacitance of the complementary switch and intra-winding capacitance of the inductor add up causing enlarged supply voltage spikes and ringing. Parasitic power supply capacitance coupled to the floating driver is also added. To make it worse, the switches are paralleled by clamping diodes due to poor quality of intrinsic body diodes. Schottky diodes, usually recommended, have particularly large junction capacitance. Yet another example is reverse energy flow, also known as power supply pumping. Power supply capacitors have to be severely oversized as to store energy returned at low frequencies. Pushing energy back and forth dramatically worsens EMI/RFI.
An instantaneously interruptible power source (I2PS) is introduced in the abovementioned xe2x80x9cPrecision Switching Power Amplifier and Uninterruptible Power System,xe2x80x9d U.S. Pat. No. 6,385,056, dated May 7, 2002, and xe2x80x9cSwitching Power Amplifier and Uninterruptible Power System Comprising DC/DC Converter for Sinusoidal Output,xe2x80x9d U.S. Pat. No. 6,362,979, dated Mar. 26, 2002. A unidirectional or bi-directional I2PS is equivalent to a conventional SPA or UPS respectively. However, some intrinsic features of the I2PS are in sharp contrast to common flaws of the conventional devices. The I2PS can instantaneously interrupt the correction, wherein a precise correction can be accomplished in every switching cycle. Moreover, the I2PS can become idle by the end of every switching cycle or remain idle over a period of many cycles. The I2PS is thus idle when no correction is necessary. If accuracy of the output voltage is nonessential, as in case of the UPS, a less frequent correction of the output voltage results in reduced power dissipation. The I2PS is unidirectional, unless otherwise noted.
The present invention is intended to provide an SPA comprising an I2PS for producing a precise AC output voltage. A converter provides an internal supply current. A fine amplifier rapidly delivers a fine current to the output capacitor. The converter and the fine amplifier can be.combined or used separately.
The switching power apparatus according to the present invention provides an AC output voltage in response to an AC input signal. A power supply means provides at least one supply voltage. A first and second inductive means attain a first and second corrective currents, and provide a first and second return voltages respectively. A first and second rectifying means limit the first and second return voltages respectively. A capacitive means provides the AC output voltage. A first switching means has a first reference terminal and a first supply terminal, and is coupled in series with the first inductive means for selectively applying the first corrective current between the power supply means and the capacitive means. A second switching means has a second reference terminal and a second supply terminal, and is coupled in series with the second inductive means for selectively applying the second corrective current between the power supply means and the capacitive means. A converter means has a first and second input terminals for converting a voltage appearing therebetween and providing a supply current to at least one supply terminal. The first input terminal is coupled to one of the terminals of the first switching means and the second input terminal is coupled to one of the terminals of the second switching means.
In another embodiment, a power supply means provides at least one supply voltage. A pair of inductive means each attain a corrective current and provide a return voltage. A pair of rectifying means is separately coupled to the inductive means for limiting the respective return voltages. A capacitive means provides the AC output voltage. A pair of switching means is separately coupled in series with the inductive means for selectively applying the respective corrective currents between the power supply means and the capacitive means. An amplifier means provides a fine current to the capacitive means in response to the AC input signal and the AC output voltage.
A voltage shifter according to the present invention provides a first binary output signal referenced to a first output potential in response to a first binary input signal referenced to an input potential and provides a second binary output signal referenced to a second output potential in response to a second binary input signal referenced to the input potential. A first current source means provides a first current in response to the first and second binary input signals. A resistive means provides a voltage referenced to the first output potential in response to the first current. A first driver means provides the first binary output signal in response to the voltage. A second current source means provides a second current in response to the voltage. A second driver means provides the second binary output signal in response to the second current.
The corrective current is equal to at least a portion of an inductive means current attained by the inductive means. The corrective current is inherently interrupted. Specifically, the respective switching means selectively applies the inductive means current to the output capacitor. When the switching means is conductive, the corrective current is equal to the inductive means current. Otherwise, the corrective current is zero. The inductive means comprises at least one inductor and/or transformer. The inductive means current is continuous if an inductor is used. Conversely, a primary current of a transformer can be interrupted. In particular, a flyback transformer provides a secondary current when the primary current is interrupted, and vice versa. Therefore, one current continues to flow in form of the other current. The corrective current recharges the output capacitor. Specifically, a current flowing through the output capacitor is equal to a difference between the corrective current and the output current of the I2PS. The latter current may be zero since no minimum load is required.
Two inductive components each attain a unidirectional current. This overcomes many disadvantages of conventional systems with a single inductive component. The inductive component attains a bi-directional inductor current that often has opposite polarity than polarity necessary to accomplish the correction. Some time has to expire before the inductor current drops to zero and builds up in the desired direction. This results in increased current ratings and higher ripple of the output voltage. Furthermore, at least one pair of complementary switches is coupled across a power supply. Each pair requires a dead time to conduct the inductive current. The dead time may be excessive in order to prevent cross-conduction of the switches. This further contributes to the increased ripple level.
The employment of the two inductive components adds other features. The currents flowing through these components are unidirectional and independently developed. The corrective current can immediately assume desired polarity and possibly an upheld level. Moreover, these currents can offset one another. The corrective current is equal to the current of one inductive component if one output switch is closed. However, the corrective current is equal to a difference between both currents if both output switches are closed. In particular, the corrective current is zero if the output switches are open or if both switches are closed and the inductive components carry even currents. The technique employing two inductive components is disclosed in the abovementioned xe2x80x9cUltra Efficient Switching Power Amplifier,xe2x80x9d U.S. Pat. No. 4,980,649 dated Dec. 25, 1990, and in both patents introducing I2PS, by the same inventor.
Continuous or discontinuous mode of operation usually applies to a conventional SPS. The mode is determined in accordance to continuity of a primary and/or secondary current. In particular, an inductive output component in the SPS carries the corrective current for correcting the output voltage. The corrective current is uninterrupted as it can only rise, fall or remain at zero level. By contrast, the corrective current is inherently interrupted in an I2PS Multiple inductive components can be used to attain the corrective current. Nevertheless, continuous or discontinuous mode of operation can be distinguished in the I2PS. Operation of each inductive output component can be divided into active and idle periods. During each active period, the respective inductive component provides at least a portion of the corrective current. Conversely, during each idle period, the corrective current is interrupted or delivered by another inductive component. The distinction between the continuous and discontinuous modes is made according to stability of the current carried by any inductive component during the idle period thereof.
In the continuous mode I2PS, energy stored in the inductive output component is maintained during each idle period. A switch or a diode is employed to effectively short the inductive component. The voltage thereacross is near zero, wherein the inductor current remains practically constant. During the active period, the corrective current immediately assumes the upheld value carried by the inductive component. The inductive component is charged only during the active period. Moreover, if multiple inductive components are employed, each inductive component is charged during that period. A lossless I2PS would be inoperative since no discharging of the inductive component would be accomplished. Inefficiencies of the real I2PS are thus taken advantage of. The continuous mode I2PS frequently operates in the discontinuous mode, e.g. near zero crossing of the output voltage. The continuous mode I2PS is inherently unidirectional unless switches are used to carry out discontinuous operation.
In the discontinuous mode I2PS, the inductive output component or components intend to be fully discharged in every switching cycle. However, the switching cycle itself is unaffected by current levels carried by the inductive components. The I2PS can thus operate in the continuous mode, particularly near peaks of the output voltage. During the idle period, energy stored in the respective inductive component is temporarily stored in a capacitor or returned to the power supply. Zero current switching improves efficiency, whereas peak currents are increased. The discontinuous mode I2PS can be bidirectional. Numerous techniques targeting discontinues mode are disclosed in the abovementioned xe2x80x9cPrecision Switching Power Amplifier and Uninterruptible Power System,xe2x80x9d U.S. Pat. No. 6,385,056, dated May 7, 2002. Therefore, the focus of the following disclosure is on the continuous mode.